1. Field of the Invention
The present invention relates to a wireless communication system capable of transmitting/receiving image data, and more particularly to an apparatus and method for efficiently transmitting/receiving image data when an error occurs during transmission/reception of the image data.
2. Description of the Related Art
In wireless communication, the first-generation (IG) communication system employs an analog scheme, and the second-generation (2G) wireless communication system employs a digital scheme in which relatively low-speed data including voice is transmitted on a wireless network. The second generation (2G) wireless-based communication system has evolved into the current third-generation (3G) communication system which provides high-speed Internet and multimedia service based on wire/wireless integration technology. Furthermore, communication systems are currently evolving from the third generation mobile communication system, known as “International Mobile Telecommunication (IMT)-2000” to a fourth-generation mobile communication system having characteristics of being ultra-high speed, handling a large quantity of data and mobility. With such development in the mobile communication system, it becomes possible for current mobile communication systems to provide multimedia service as well as voice service, contrary to the conventional mobile communication system providing only voice service.
The multimedia service provided to users can handle video data, audio data and especially, image data, which include a great amount of data. For this reason, various methods for transmitting image data are being proposed. The transmission of the image data will be described hereinbelow with reference to a typical Wideband Code Division Multiple Access (WCDMA) system, from among mobile communication systems providing the multimedia service, as an example.
The WCDMA system employs the H.324M standard in order to perform image communication, i.e., to transmit/receive the image data. The H.324M standard refers to moving-picture communication including video, audio, etc. in a wireless channel. The recommended standard for H.324M includes the H.223 standard in relation to multiplexing, the H.263 standard for encoding video signals, the MPEG-4 (Moving Picture Experts Group standards 4), an Audio/Modem Riser (AMR) for encoding audio signals, a G.723.1 voice encoder, and the H.245 standard for encoding control data. In addition, video, audio and control data are encoded by using the above-mentioned standards, and then the encoded data are multiplexed/demultiplexed in a protocol data unit (PDU) according to the H.223 standard which is a standard for multiplexing. Before a multiplexing procedure in a PDU is performed, a sequential number and a cyclic redundancy check (CRC) for detecting an error occurring in a data transmission procedure according to the H.223 standard is applied in an adaptation layer. When a frame generated according to the H.223 standard is demultiplexed, it is determined if there is an error by using the sequential number and the CRC included in the frame.
A WCDMA system transmitting/receiving image data using the H.324M standard as described above includes user equipments (UEs) for transmitting/receiving the image data, a serving node B for providing service to the UEs, and a plurality of neighbor node Bs. In addition, communication routes for performing data communication through the node B are secured between the UEs, and an image data service is provided through the communication routes.
The WCDMA system currently uses I frames and P frames in order to transmit/receive the visual image. A procedure for transmitting/receiving image data by using such image frames will now be described with reference to FIG. 1. FIG. 1 is a flow diagram schematically illustrating a procedure for transmitting/receiving image data in a typical WCDMA system.
An I frame is a whole image frame having data for an entire-image (i.e., information about one entire image for reproduction of image data) when image data are transmitted. Therefore, when using such an I frame, the UE can reproduce one entire image. However, the I frame includes a great amount of data. Therefore, not just the I frames are transmitted, but a predetermined number of P frames are transmitted between the I frames. That is, data for a partial image, i.e., compressed data for reproducing image data, is transmitted between the I frames. The compressed data includes information about a changing portion as compared with a previously-reproduced image, which is called a ‘P frame’. When the UE receives the P frame, the UE reproduces an image by applying the change of an image included in the received P frame to a previously-reproduced image.
As described above, in the current WCDMA system, information about the image change is transmitted by interposing and transmitting the P frame between the I frames transmitted at a predetermined interval. This method is employed to decrease the transmission/reception quantity of large-scale image data and to increase a transmission efficiency.
In FIG. 1, the WCDMA system includes ‘UE 1’ 100 and ‘UE 2’ 150, in which a procedure for transmitting image data from ‘UE 1’ 100 to ‘UE 2’ 150 is shown. In ‘UE 1’ 100, image data are subjected to an encoding procedure by an encoder 110. The image data having been subjected to the encoding procedure are processed according to the H.324M standard. Image frames, i.e., I frames and P frames, generated with the image data are multiplexed and transmitted to a wireless network. First, ‘UE 1’ 100 transmits an I frame 101 for reproducing image data, and then ‘UE 2’ 150 receives the I frame 101 in accordance with the H.324M standard. Thereafter, ‘UE 2’ 150 performs a demultiplexing procedure for the received I frame 101 and checks if corresponding image data include an error. In this case, ‘UE 2’ 150 checks if there is an error in the received image data by using CRC information and sequential number inserted in the transmission frame.
The image data are reproduced by a decoder 120 included in the ‘UE 2’ 150. After transmitting the I frame 101, ‘UE 1’ 100 transmits a P frame. In this case, the transmitted P frame includes information only about changes from a previous frame.
However, since data communication between ‘UE 1’ 100 and ‘UE 2’ 150 is performed through a wireless network, an error may occur in the image data due to various factors, such as attenuation and distortion of a signal, noise, etc., in the wireless network. In this case, ‘UE 2’ 150 receiving the image data determines if there is a loss or an error in the image data. ‘UE 2’ 150 checks if there is a transmission error in the received image frames (i.e., I frame and P frame) by using the CRC information and the sequential number, and does not decode a corresponding image frame when there is a transmission error. When a P frame 103 transmitted from ‘UE 1’ 100 includes an error, ‘UE 2’ 150 does not perform a decoding procedure for the P frame 103. Thereafter, when ‘UE 2’ 150 receives the next P frame 105 having no error, the ‘UE 2’ 150 reproduces image data by decoding the P frame 105.
Then, ‘UE 2’ 150 checks if there is an error in the following image frames transmitted from ‘UE 1’ 100 for each of the image frames. When an error occurs in an image frame, the ‘UE 2’ 150 does not perform a decoding procedure for the relevant image frame. Therefore, if an error occurs in the P frame 103, the ‘UE 2’ 150 does not decode the P frame 103. Then, ‘UE 2’ 150 decodes the P frame 105 received after the P frame 103, if there is no error. In this case, since the received P frame 103 has not been decoded, an abnormal image is reproduced when the P frame 105 is received and reproduced, since the data from P frame 103 is missing.
Accordingly, when image data are transmitted using the above-mentioned conventional manner, image data may not be normally restored with only P frames which follow an image frame containing an error unless the entire-image frame (i.e., an I frame) is received. As a result, the conventional method has a problem in that an abnormal image is reproduced in a UE receiving the image data. In addition, when an error in image data occurs, a user of the relevant UE can not avoid viewing abnormal images when a P frame containing the error is received as compared to when an I frame is received.